1. Field of the Invention
This invention relates generally to materials for shielding against nuclear radiation and more particularly, against neutrons and gamma rays (also referred to as photons).
2. Description of Related Art
The need to shield against nuclear radiation for protection of personnel and instruments is encountered in a wide variety of installations and activities. These installations include but are not limited to fission reactors, fusion facilities, and particle accelerators. The activities include, but are not limited to research, medical applications, oil logging, as well as transport and storage of radioactive materials.
Shielding against neutrons usually involves the following processes: (a) Slowing-down of the neutrons; that is, reducing the neutrons' kinetic energy. This is achieved by letting the neutrons undergo elastic, and sometimes also inelastic scattering collisions with the nuclei of atoms of different materials which constitute the shield. (b) Absorption of the neutrons. In general, as the neutron energy becomes lower, the probability increases for the neutron to be absorbed by the nuclei of the shield constituents, while traversing a given distance in the shield. Most of the neutron absorption reactions and neutron inelastic scattering collisions give off photons, usually referred to as secondary photons (to distinguish them from photons which do not originate from neutron interactions within the shield). (c) Absorption of the secondary photons.
It is well known that hydrogen is very effective for the slowing-down of neutrons. This is due to the relatively large fraction of its energy that a neutron loses, on the average, when it collides (elastically) with a proton--the nucleus of an hydrogen atom. Hence, hydrogen is one of the major constituents of many materials used for neutron shielding, i.e., for shielding against neutron radiation. Henceforth, materials containing a substantial amount of hydrogen will be referred to as hydrogenous materials.
Most of the materials used for radiation shielding applications are in the solid state. Among the most effective solid hydrogenous materials for shielding against neutron radiation are organic polymers, such as plastics and rubbers. Examples include, but are not limited to hydrocarbon plastics (such as polyethylene, polypropylene and polystyrene); natural and synthetic rubber (such as silicone rubber); and other plastics or resins containing atoms in addition to hydrogen and carbon (such as acrylic, polyester, polyurethanes and vinyl resins). The high effectiveness of these organic polymers for shielding against neutrons is due to the large concentration of hydrogen atoms in these materials. A common measure of the concentration of hydrogen is the hydrogen atomic density--the number of hydrogen atoms in a cubic centimeter of the given material.
One of the organic polymers most widely used for radiation shielding is polyethylene. Its hydrogen atomic density is nearly 8.times.10.sup.22 atoms/cm.sup.3 ; this is higher than the hydrogen density in most of the other organic polymers. It is about 20% higher than the atomic density of hydrogen in water, when both are at room temperature. Henceforth we shall explicitly refer to polyethylene; it is to represent other hydrogenous materials as well.
As a result of its high hydrogen atomic density, polyethylene is considered to be one of the most effective materials for the attenuation of neutrons having an energy of few MeV or lower. One measure of the neutron attenuation ability of a shielding material is the reduction in the energy the neutrons leaking out from a shield of a given thickness will deposit per unit weight of a biological tissue per unit time--referred to in the profession as "dose rate". The neutron attenuation ability of a shield is to be distinguished from "neutron shielding ability" which is a measure of the ability of the shield to reduce the combined contribution to the dose rate of the neutrons and their secondary photons. Similarly, the photon attenuation ability of a shielding material will be measured by the ability of a given thickness of this material to reduce the contribution of a given source of photons to the dose rate behind the shield. As the probability for neutron-producing photon reactions is very small, the term "photon shielding ability" is practically identical to the photon attenuation ability.
One drawback of pure polyethylene as a shielding material against neutrons is that the slowed-down neutrons which are absorbed by the hydrogen of the polyethylene generate a significant source of 2.2 MeV photons (i.e., photons having an energy of 2.2 Million electron Volts). The neutron shielding ability of polyethylene can be improved if a good neutron absorbing material is added to it so as to reduce the production probability of energetic secondary photons. The neutron absorbing material in common use is boron. Thus, for example, one of the most commonly used polyethylene based commercial shielding materials against neutron sources is borated polyethylene. One example of such a commercial shielding material is the 5% Boron-Polyethylene being offered by Reactor Experiments, Inc. (to be referred to as R/X)--R/X Catalog number 201. (This catalog can be obtained from Reactor Experiments, Inc., 1275 Hammerwood Ave., Sunnyvale, Calif. 94089-2231).
Another drawback of polyethylene as a shielding material is that it has a relatively low specific weight, also referred to as density. In general, the higher the density of a material, the better is its shielding ability against energetic photons (defined, for example, as photons having energy greater than 1 MeV) for a given shield thickness. In other words, given two shields of identical geometry, the shield made of the higher density material will, in general, attenuate the energetic (and more difficult to attenuate) photons better than the shield made of the lower density material. The specific weight or density of a material is commonly measured by the number of grams of this material which occupy one cubic centimeter. A typical density of polyethylene is 0.92 g/cm.sup.3. For comparison, a typical density of ordinary concrete is 2.3 g/cm.sup.3, of stainless steel is 7.8 g/cm.sup.3, of lead is 11.3 g/cm.sup.3, and of tungsten is up to 19.2 g/cm.sup.3.
Due to its relatively low density, the neutron shielding ability of pure polyethylene and even of borated polyethylene is not very good; the relatively energetic (2.2 MeV) secondary photons are not well attenuated by the polyethylene. Thus, the secondary photon contribution to the dose rate beyond a pure polyethylene shield can far exceed the contribution of the neutrons to the dose rate. Similarly, pure or borated polyethylene are not, by themselves, good shielding materials for applications in which the shield is exposed to photons as well as to neutrons. In other words, pure and borated polyethylene (and similar low density hydrogenous materials) have a superb attenuation ability for neutrons having energies up to a few MeV, but poor neutron shielding ability and poor photon shielding ability.
Consequently, when shielding against neutron sources, or combined neutron and photon sources, it is customary to use a two-layer shield--one layer made of polyethylene and the other layer made of a higher density material, such as concrete or lead. It is also customary to use a single layer shield made of a material which features a suitable combination of hydrogen atomic density and specific weight.
One approach to the production of hydrogenous materials having densities higher than polyethylene is to add to the polyethylene a higher density constituent. A common additive used for improving the photon attenuation ability of polyethylene is lead. Thus, for example, the polyethylene based commercial shielding materials against a combined radiation of neutrons and photons being offered by Reactor Experiments, Inc. is Poly-Boron Lead; R/X Catalog Number 202. The boron is added for the purpose of absorbing the slowed-down neutrons and, thus, suppressing the secondary photon source.
One drawback of lead loaded borated polyethylene is that its attenuation ability for fission neutrons and for lower energy neutrons is inferior to that of pure and borated polyethylene. Another drawback of lead loaded shielding materials is that lead is a chemically toxic element, and its use is becoming more and more restricted and complicated. Yet another drawback of lead loaded borated polyethylene is that lead is more expensive per unit volume than polyethylene. Thus, the price of R/X material No. 202 is more than 3 times higher than the price of an equal volume of R/X material No. 201.
A completely different approach to the design of shields against a combined radiation of neutrons and photons is to use concretes. The composition and density of concretes used for radiation shielding can vary significantly from application to application. Information on different concretes can be found in Sections 9.1.12 and 9.1.19 of Volume II (entitled "Shielding Materials") of the "Engineering Compendium on Radiation Shielding", R. G. Jaeger, Editor-in-Chief, Springer-Verlag, New-York (1975).
The density of conventional concretes is, typically, 2.3 g/cm.sup.3. The atomic density of hydrogen in conventional concretes is, typically, 1.5.times.10.sup.22 atoms/cm.sup.3. This is only about 20% of the hydrogen atomic density in polyethylene. Due to their relatively low hydrogen atomic density, conventional concretes do not make good neutron shields. For applications in which shielding against neutrons is an important design goal, and a concrete type material is desirable, it is possible to use special, more expensive concretes which can hold a larger amount of water than ordinary concretes. Examples of such concretes can be found in the above identified Volume II of the "Engineering Compendium on Radiation Shielding".
A drawback of the high hydrogen density concretes is that their hydrogen atomic density, although higher than that of ordinary concretes, is still much lower than in high hydrogen density hydrogenous materials such as polyethylene. Other types of commercially available shielding materials which can be cast similar to concretes, but which have higher hydrogen atomic density than concretes, are made of small particles or beads of some plastic material mixed with a cementitious material. One example of such a shielding material is the so called POLY/CAST, material No. 259 in the catalog of R/X. It uses a cementitious material to bond the beads of polyethylene. Hall and Peterson, in U.S. Pat. No. 4,123,392, entitled "Non-Combustible Nuclear Radiation Shields with High Hydrogen Content," suggest using either Portland cement, wall plaster, plaster of Paris, silica gel or clay for bonding many different types of hydrogenous materials. The motivation of Hall and Peterson to bond hydrogenous materials by such cementitious materials was to reduce the fire hazard of the hydrogenous materials.
One drawback of POLY/CAST is that it has a low density of 1.15 g/cm.sup.3 ; this is only slightly higher than the 0.92 to 0.96 g/cm.sup.3 of pure polyethylene. Thus, it will not provide an effective shielding against photons. Another drawback of POLY/CAST is that it can be chipped off easily and can develop cracks. Thus, it is not being used for structural or stand-alone components. Rather, it is usually cast into a mold of some kind, such as a stainless steel container. Still another drawback of POLY/CAST is that it is not machinable. Yet another disadvantage of POLY/CAST is that it is not reusable. The latter two drawbacks also apply to concretes of all kinds.
The non-reusability of POLY/CAST is due to the fact that it can not be reshaped as easily as polyethylene or polyethylene bonded materials. One way to reshape the latter is by melting and recasting them--essentially repeating the original process of their fabrication. This is because bonding by polyethylene is physical; it can be obtained, for example, by mixing molten polyethylene with the particles to be bonded, and letting the polyethylene solidify. On the other hand, bonding by cementitious materials such as Portland cement and plasters of various types is based on chemical processes--chemical reactions between the cement constituents and water.
An additional drawback of cementitious materials (including concretes and POLY/CAST) is that the amount of water that they bond when being cast tends to decrease with time. This implies that the atomic density of the hydrogen contained in these materials may decrease with time. This will impair the neutron shielding ability of these shielding materials.
It is well known to those skilled in the art that a proper combination of hydrogenous and good inelastically scattering materials can slow down and, therefore, attenuate high energy neutrons better than either pure polyethylene or the inelastic scattering material by themselves. Inelastically scattering materials are materials with which high energy neutrons can undergo collision reactions which result in a conversion of part of the colliding neutron kinetic energy to the internal energy of the nucleus of the material with which the neutron collided. Inelastic scattering by materials such as tungsten, lead and even iron is known to be a more effective mechanism than elastic scattering by hydrogen for slowing down neutrons having energies higher than a few MeV.
It is also well known to those skilled in the art that the higher the neutron energy, the larger becomes the inelastic scattering contribution to the neutron slowing down. Thus, whereas the lead borated polyethylene of R/X catalog number 202 is less effective than pure or borated polyethylene for the attenuation of fission-born neutrons, it is more effective than pure or borated polyethylene for the attenuation of fusion-born neutrons. Herein "fission-born neutrons" (or "fission neutrons") are neutrons which are emitted from fission reactions; their average birth energy is approximately 2 MeV. "Fusion-born neutrons" (or "fusion neutrons") are neutrons which are emitted from the fusion of one deuterium nucleus and one tritium nucleus. These neutrons' birth energy is close to 14 MeV. Deuterium and tritium are isotopes of hydrogen; they differ from protium--the most abundant hydrogen isotope--by the number of neutrons in their nuclei: 1 and 2 neutrons in, respectively, the nuclei of deuterium and tritium, versus no neutron in the nucleus of the protium. Henceforth, the term "high energy neutrons" will refer to neutron from a source with average energy that is higher than the average energy of fission neutrons. It includes, but is not limited to fusion neutrons. The term "fission-like neutrons" will refer to neutrons from a source with average energy that is equal to or lower than 2 MeV. The term "neutrons" will refer to neutrons of any energy.
With few exceptions, the lower the atomic mass number of the element, the less effective the element is as an inelastic scatterer. Thus, materials such as magnesium, silicon, calcium, fluorine and oxygen are usually considered to be poor inelastic scatterers as well as poor elastic scatterers. Consequently, the constituents of concretes and cementitious materials other than water (which is added to harden them) are considered poor inelastic scatterers as well as poor elastic scatterers of neutrons. There is no known teaching in the background art that a combination of such materials with polyethylene or another hydrogenous material can provide a better neutron attenuation ability that the hydrogenous material by itself.